An alternative scale of judging female attractiveness that is similar to black jack. The lower the number you give the female, the more times that you would like to have intercourse with or "hit" the female. Inversely, higher numbers are reserved for females that are rather unattractive and in most situations in which you are not piss drunk, you would not want to have intercourse with or "hit" the female.

Damn that girl is fine! She's definitely a 1 on the Black Jack Scale. I'd hit her again and again and again!
Holy shit that girl is nasty! I'd never do her, she's a 21 on the Black Jack Scale no doubt